Editing a rigged mesh (MakeHuman > Blender > Unity)
I think your steps should look more like the following:


Edit the mesh

1.1 - delete the vertices for the head

1.2 - shift+a (to add) and select cube (while still in edit mode)

1.3 - The cube should be selected. Place the cube (g button) to desired location.

1.4 - make sure the vertices of the cube are still selected, and go into the "data" panel (icon is a polygon triangle thingy), find the vertex group named "DEF-head", and assign the cube vertices to that group (set slider to 1.0 and click "Assign" button).